Grain size dependency of cosmogenic nuclide concentrations in alluvial sediment in an arid zone catchment.

Be and Al concentrations in the 15 individual pebbles range from ∼0.2 to ∼22.7 x 10 atoms.g−1 and ∼1.3 to ∼72.8 x 10 atoms.g−1, respectively. When amalgamated, the pebbles yield average Be and Al concentrations of ∼6.7 and ∼27.3 x 10 atoms.g−1, respectively. These average concentrations yield minimum and maximum Be model erosion rates of ∼0.4 and ∼2.1 m.Myrs−1, and minimum and maximum Al model erosion rates of ∼0.3 and ∼1.4 m.Myrs−1, respectively. In contrast, the amalgamated sand yields an average Be concentration of ∼0.8 x 10 atoms.g−1, and associated minimum and maximum Be model erosion rates that are an order of magnitude larger than those obtained for the amalgamated pebbles (i.e., ∼4.8 and ∼13.0 m.Myrs−1, respectively). Modelling results suggest that a difference in sediment transport times of the order of 10-10 years is necessary to explain the difference in cosmogenic nuclide inventories between the pebble and sand samples. Given the small catchment size and lack of accommodation space, such long transport times are unrealistic for the Gaub catchment. Furthermore, the Al/Be ratios in the pebbles are indicative of simple exposure histories, suggesting that burial, and thus, storage of the pebbles has not been substantial. Therefore, the difference in nuclide concentrations between the pebble and sand samples cannot be caused solely by longer sediment residence times for the pebbles than for the sand grains.